Emulsions may be formed when two or more immiscible liquids, usually water or a water-based solution and a hydrophobic organic liquid (e.g., an oil), are mixed so that one liquid forms droplets in the other liquid. Either of the liquids can be dispersed in the other liquid. When, for example, oil is dispersed in water, the emulsion may be referred to as an oil-in-water (o/w) emulsion. The reverse case is a water-in-oil (w/o) emulsion. More complex emulsions such as double emulsions may be formed when, for example, water droplets in a continuous oil phase themselves contain dispersed oil droplets. These oil-in-water-in-oil emulsions may be identified as o/w/o emulsions. In the same manner a w/o/w emulsion may be formed.
A problem with many emulsions is that if they are not stabilized, for example, by adding surfactants or emulsifiers, they tend to agglomerate, form a creaming layer, coalesce, and finally separate into two phases. If a surfactant or emulsifier (sometimes referred to as a surface-active agent) is added to one or both of the immiscible liquids, one of the liquids may form a continuous phase and the other liquid may remain in droplet form (“dispersed or discontinuous phase”), the droplets being dispersed in the continuous phase. The degree of stability of the emulsion may be increased when droplet size is decreased below certain values. For example, a typical o/w emulsion of a droplet size of 20 microns may be only temporally stable (hours) while that of one micron may be considered as “quasi-permanently” stable (weeks or longer). However, the energy consumption and the power requirement for the emulsification system and process may be significantly increased for smaller droplet sizes when using conventional processing techniques, especially for highly viscous emulsions with very small droplet sizes and large outputs. For example, the doubling of energy dissipation (energy consumption) may cause a reduction of average droplet size of only about 25% when using conventional processing techniques. Shear force may be applied to overcome the interfacial tension force and in turn to break larger droplets into smaller ones. However, as the droplet size decreases, the interfacial tension required to keep the droplet shape tends to increase. Energy consumption may take place in various forms, for example, it can be the energy needed by the stirrer to overcome shear force of the emulsion in a batch process, the energy for heating and cooling, and/or the power to overcome pressure drop in a continuous process such as in a homogenizer. Heating is often needed for emulsification when one of the phases does not flow or flows too slowly at room temperature. A heated emulsion typically has lower stability, however, due to lower viscosity of the continuous phase and in turn less drag. Drag may be necessary to stop or resist the motion of the droplets and in turn the coalescence into larger and often undesired droplets or aggregates of droplets as well as phase separation into layers. After emulsification, droplets tend to rise by buoyancy. As such, an immediate cooling down may be needed, which also consumes energy.
A problem with many of the processes that are currently available for making emulsions is that the range of compositions that are feasible for formulating product are constrained. For example, a problem with many of the emulsions that are currently available relates to the presence of surfactants or emulsifiers in their formulations. These surfactants or emulsifiers may be required to stabilize the emulsions, but may be undesirable for many applications. For example, heating without bubbling or boiling is often desired in emulsification processes, however in some instances the onset temperature of nucleate boiling or air bubble formation from dissolved air in the continuous phase may lower when surfactants or emulsifiers are present. Boiling may cause unwanted property changes. Air bubbles may cause creaming and other undesired features.
Emulsions that have low surfactant or emulsifier concentrations or are free of such surfactants or emulsifiers are often desirable for skin care products in the cosmetic industry. A disadvantage with some surfactants or emulsifiers is their tendency to interact with preservatives, such as the esters of p-hydroxybenzoic acid, used in skin care products. Skin irritation is another problem often associated with the use of surfactants or emulsifiers. Many adverse skin reactions experienced by consumers from the use of cosmetics may be related to the presence of the surfactants or emulsifiers. Another example relates to the problem with using surfactants or emulsifiers wherein water proofing is desired. For example, in water-based skin care products such as sunscreen, the active ingredient may not be waterproof due to the presence of water-soluble surfactants or emulsifiers.
A problem relating to the use of many pharmaceutical compounds relates to the fact that they are insoluble or poorly soluble in water and there are limitations as to the surfactants or emulsifiers that can be used. This has resulted in the discovery of drugs that are not clinically acceptable due to problems relating to transporting the drugs into the body. Emulsion formulation problems may be problematic with drugs for intravenous injection and the administration of chemotherapeutic or anti-cancer agents.